Prevent save a record - New forms

I don't know if that kind of form rule based on file names in New Style forms is supported yet.  You would need to check with support.  But you could put in n a data table rule to look for that condition and block the save.  That is a setting in Advanced settings for the table. Basically you construct an error message to the user in a text formula and if that does not calculate to blank (null), then the record will not save.

I contacted support before open discussion and 
they says me me this can't posible in the new version forms. And they suggested use this channel to found a way to implement this.

Ok, so you can try"Custom Data Rules".  Look for it in Advanced Settings for the table.

Sorry I don't comment the support team give me an answer about it... When I started this discussion.

wow! Really can I handle this with the dynamic table rule. Here's the implementation.

Is there anything you can suggest to handle and complete the formula for me.

I need evaluate between the 3 attachments, so basically I need to include the multiple combinations for the 3 attachments. 

Ok, so before we build the Custom Data Rule, let's just make a regular formula text field to calculate the error message.  Are you saying that there are three fields for attachments and no two of them can be the same? Can you tell me the three field names?

Actually, there cannot be any duplicate attachments. the name of fields are, Evidence, Customer invoice, Purchase order attach. 

Thank you very much, for helping me to solve this.!!!